This invention relates generally to combine harvesters and, more particularly, to such machines fitted with some form of control system operable to adjust the amount of crop material passing through the machine by varying the ground speed of the combine.
The term "grain" will be used principally throughout this specification for convenience and it should be understood that the term is not intended to be limiting. Thus "grain" refers to that part of the crop which is threshed and separated from the discardable part of the crop material.
The purpose of control systems for combine harvesters is to maximize the capacity of the machine and yet minimize grain loss. Numerous control systems have been proposed but to date, no system has been commercially adapted for combine harvesters primarily because all systems so far proposed are impracticable in the sense that they do not meet the stated objective in a wholly satisfactory manner. The main factor affecting the practicability of combine harvester control systems is that of grain loss. On the one hand, a control system which does not take account of grain loss cannot seriously be entertained because grain loss is a highly critical factor in the harvesting operation. On the other hand, a control system which does take account of grain loss is suspect in that there is presently no method of measuring grain loss in absolute terms. Accordingly, any system relying on a grain loss measurement lacks the required accuracy.
Another problem with utilizing some form of grain loss measurement to effect machine control is that the control loop is relatively slow because a given volume of crop material takes a finite time (typically 10 seconds in conventional combine harvesters and approximately 5 seconds in rotary combine harvesters) to progress through the machine before the grain loss associated with that volume is measured. With such a slow control loop, the control system can become unstable by oscillating between maximum and minimum controls. This problem is also associated with any other parameter of the machine which may be used in the control system and which is related to some crop processing component located in the machine such that there is a significant time delay between crop material entering the machine and the reaching of that component.
Many of the control systems proposed to date adjust the ground speed of the combine harvester in order to control the intake of crop material (i.e. to control machine loading) but other bases of control may be used.